I've only seen summaries of the ending, but I like to think that what really killed Starkiller was stopping to contemplate. It was actually the Light Side that was his downfall.

It's either that or assume "bad writers who forgot that it's entirely possible to kill people while remaining on the Light Side, even in anger, and killing probably the most evil being in the universe would definitely qualify". Go hide in a swamp later if you can't face it, but just get 'er done.